Dr David Clarke, author of The UFO files and Senior Lecturer in Journalism at Sheffield Hallam University, has written an introduction to the files. He said many of the sightings may have been influenced by what people had read and watched.

"In the 1950s the next big leap in technology was thought to be a round craft that took off vertically, and it's intriguing to note that this is the same period when people began to report seeing 'flying saucers' in the sky,'" he suggested.

"In the period the latest file release covers (1994-2000), triangular-shaped US stealth bombers and Aurora spy planes featured heavily on TV (The X Files 1993-2002) and films (Independence Day, 1996) and the shape of reported UFOs corresponds.

"It's impossible to prove a direct link between what people are reading and watching and what they report as UFOs, but one interpretation could be that the latest advances in technology may be influencing what people see in the sky."

The files are available to download for free for a month from the National Archives website. The site also hosts a videocast dramatising highlights from the files alongside a detailed guide to researching the files by Dr Clarke.